Fix code example in testing-support-library.

Note: mDevice is a variable, there is additional brackets.

Bug:
https://code.google.com/p/android/issues/detail?id=206043

Change-Id: I8e88a6144f1ede1d45262ff034159de524c24b70
(cherry picked from commit 6b1853008e)

docs/html/topic/libraries/testing-support-library/index.jd

@@ -523,7 +523,7 @@ onView(withId(R.id.changeTextBt)).perform(click());</pre>
 mDevice = UiDevice.getInstance(getInstrumentation());
 
 // Perform a short press on the HOME button
-mDevice().pressHome();
+mDevice.pressHome();
 
 // Bring up the default launcher by searching for
 // a UI component that matches the content-description for the launcher button